One typical prior art method for preparing polyorganosiloxane resins is by hydrolytic condensation of organohalogenosilanes. Reaction control is difficult since organohalogenosilanes are highly reactive. To produce a polyorganosiloxane resin having desired properties, it is necessary to use a vast volume of solvent or water or even harmful solvents such as trichloroethylene. Hydrohalogenic acid by-products and disposal of organic waste water are outstanding problems.
Another known method is by hydrolytic condensation of organoalkoxysiloxanes. This method is easy in reaction control and thus advantageous over the former method starting with organohalogenosilanes. A problem arises with a large amount of alcohol by-products. When the reaction is terminated by water washing, there is left a large amount of alcoholic waste water.
As a solution to this problem, it is known in the art to remove the alcohol by-product by distillation. However, if distillation takes place without removing or deactivating the hydrolytic condensation catalyst, silanol groups often undergo condensation to form a gel. Therefore, the polyorganosiloxane resins that can be produced in a stable state by this method are limited to those having only alkoxy groups as a functional group and a relatively low molecular weight. It is possible to deactivate the hydrolytic condensation catalyst by neutralization before distillation. Even in this case, heating and concentration during distillation can cause condensation of functional groups to alter the functional group content, viscosity, and molecular weight distribution. It is thus difficult to avoid any influence on the properties of the resulting polyorgano. siloxane resin. The following reasons are presumed. Since pH control in proximity to a neutralization equivalent point is difficult, a significant change of pH can occur with a slightly excess or short amount of neutralizing agent. Since neutralization reaction in organic solvent often takes a long time, considerable amounts of acid and alkali are left unreacted.